Abstract
The complex inheritance of resistance to Cercospora leaf spot (CLS), the most severe fungal foliar disease in sugar beet, was investigated by means of quantitative trait loci (QTL) analysis. Over a three year period, recombinant inbred lines (RILs) of sugar beet (Beta vulgaris L.), generated through a cross between lines resistant (‘NK-310mm-O’) and susceptible (‘NK-184mm-O’) to CLS, were field-tested for their resistance to the pathogen. Composite interval mapping (CIM) showed four QTL involved in CLS resistance to be consistently detected. Two resistant QTL (qcr1 on chromosome III, qcr4 on chromosome IX) bearing ‘NK-310mm-O’ derived alleles promoted resistance. Across 11 investigations, the qcr1 and qcr4 QTL explained approximately 10% and over 20%, respectively, of the variance in the resistance index. Two further QTL (qcr2 on chromosome IV, qcr3 on chromosome VI) bearing ‘NK-184mm-O’ derived alleles each explained about 10% of the variance. To identify the monogenic effect of the resistance, two QTL derived from ‘NK-310mm-O’ against the genetic background of ‘NK-184mm-O’, using molecular markers. The qcr1 and qcr4 were precisely mapped as single QTL, using progenies BC5F1 and BC2F1, respectively. The qcr1 that was located near e11m36-8 had CLS disease severity indices (DSI) about 15% lower than plants homozygous for the ‘NK-184mm-O’ genotype. As with qcr1, heterozygosis of the qcr4 that was located near e17m47-81 reduced DSI by about 45% compared to homozygosis. These two resistant QTL might be of particular value in marker-assisted selection (MAS) programs in CLS resistance progression.
Highlights
The complex inheritance of resistance to Cercospora leaf spot (CLS), the most severe fungal foliar disease in sugar beet, was investigated by means of quantitative trait loci (QTL) analysis
As a part of this program, we focused our analysis on the genetics of resistance to CLS
The objectives of this study were to: (1) identify QTL for CLS resistance within our breeding material under field conditions, (2) resolve the QTL into individual genetic factors using recombinant inbred lines (RILs), and (3) evaluate the genetic effect of each QTL using near isogenic lines (NILs) produced by recurrent back crosses
Summary
The complex inheritance of resistance to Cercospora leaf spot (CLS), the most severe fungal foliar disease in sugar beet, was investigated by means of quantitative trait loci (QTL) analysis. Two QTL analyses conducted by Setiawan et al (2000)—one based on a field test under natural infection condition and another using a leaf disk test—detected at least four QTL While these studies were pivotal in elucidating the genetics of CLS resistance in sugar beet, questions remained as to the precise map positions of the QTL, as well as their respective gene products and effects. An initial field-based screening of Japanese sugar beet lines identified breeding line ‘NK-310mm-O’ as a source of a high level of resistance to multiple diseases (Taguchi et al 2007) This prompted us to characterize the genetic nature of these resistance traits, with a goal of establishing a marker-assisted selection (MAS) system for resistance to these diseases. The objectives of this study were to: (1) identify QTL for CLS resistance within our breeding material under field conditions, (2) resolve the QTL into individual genetic factors using recombinant inbred lines (RILs), and (3) evaluate the genetic effect of each QTL using near isogenic lines (NILs) produced by recurrent back crosses
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